Characterization of conjugated metabolites of benzo[a]pyrene in germ-free rat urine by liquid chromatography/electrospray tandem mass spectrometry

Metabolism and excretion of 1-hydroxymethylpyrene, the proximate metabolite of the carcinogen 1-methylpyrene, in rats

1-Methylpyrene, an alkylated polycyclic aromatic hydrocarbon and environmental carcinogen, is activated by side-chain hydroxylation to 1-hydroxymethylpyrene (1-HMP) and subsequent sulfo conjugation to the DNA-reactive 1-sulfooxymethylpyrene. In addition to the bioactivation, processes of metabolic detoxification and transport greatly influence the genotoxicity of 1-methylpyrene. For a better understanding of 1-HMP detoxification in vivo we studied urinary and fecal metabolites in rats following intraperitoneal doses of 19.3mg 1-HMP/kg body weight (5 rats) or the same dose containing 200μCi [(14)C]1-HMP/kg body weight (2 rats). After 48h, 48.0% (rat 1) and 29.1% (rat 2) of the radioactivity was recovered as 1-HMP in the feces. Six major metabolites were observed by UV and on-line radioactivity detection in urine samples and feces after HPLC separation. The compounds were characterized by mass spectrometry, (1)H NMR and (1)H-(1)H COSY NMR spectroscopy, which allowed assigning tentative molecular structures. Two prominent metabolites, 1-pyrene carboxylic acid (M-6) and the acyl glucuronide of 1-pyrene carboxylic acid (M-5) accounted for 17.7% (rat 1) and 25.2% (rat 2) of the overall radioactive dose. Further, we detected the acyl glucuronide of 6-hydroxy-1-pyrene carboxylic acid (M-1) and 8-sulfooxy-1-pyrene carboxylic acid (M-3) together with two regioisomers of M-3 (M-2 and M-4) differing in position of the sulfate group at the pyrene ring. In urine samples, the radioactivity of 1-pyrene carboxylic acid and its five derivatives amounted to 32.4% (rat 1) or 45.5% (rat 2) of the total [(14)C]1-HMP dose.

Effects of Nereis diversicolor on the transformation of 1-methylpyrene and pyrene: transformation efficiency and identification of phase I and II products

Transformation of nonsubstituted and alkyl-substituted polycyclic aromatic hydrocarbons (PAHs) by the benthic invertebrate Nereis diversicolor was compared in this study. Pyrene and 1-methylpyrene were used as model compounds for nonsubstituted and alkyl-substituted PAHs, respectively. Qualitative and quantitative analyses of metabolites and parent compounds in worm tissue, water, and sediment were performed. Transformation of 1-methylpyrene generated the benzylic hydroxylated phase I product, 1-pyrenecarboxylic acid that comprised 90% of the total metabolites of 1-methylpyrene, and was mainly found in water extracts. We tentatively identified 1-methylpyrene glucuronides and 1-carbonylpyrene glycine as phase II metabolites not previously reported in literature. Pyrene was biotransformed to 1-hydroxypyrene, pyrene-1-sulfate, pyrene-1-glucuronide, and pyrene glucoside sulfate, with pyrene-1-glucuronide as the most prominent metabolite. Transformation of 1-methylpyrene (21% transformed) was more than 3 times as efficient as pyrene transformation (5.6% transformed). Because crude oils contain larger amounts of C₁-C₄-substituted PAHs than nonsubstituted PAHs, the rapid and efficient transformation of sediment-associated 1-methylpyrene may result in a high exposure of water-living organisms to metabolites of alkyl-substituted PAHs, whose toxicities are unknown. This study demonstrates the need to consider fate and effects of substituted PAHs and their metabolites in risk assessments.

Biotransformation of benzo[a]pyrene in Ahr knockout mice is dependent on time and route of exposure

Benzo[a]pyrene (BP) is an ubiquitous environmental pollutant with potent mutagenic and carcinogenic properties. The Ah receptor (Ahr) is important in the metabolic activation of BP and is therefore central to BP-induced carcinogenesis. Although Ahr(-/-) mice are refractory to BP-induced carcinogenesis, higher levels of BP-DNA and -protein adducts were formed in them than in wild-type mice. These results indicated the presence of an Ahr-independent and/or a slower biotransformation of BP in Ahr knockout mice. To address this issue further, we have now performed a time-course experiment, with mice receiving a single oral dose of BP (100 mg/kg). Wild-type mice have an effective clearance of BP metabolites, mainly through 3-hydroxybenzo[a]pyrene and 9-hydroxybenzo[a]pyrene in the feces with reduced levels of DNA and protein adducts in the examined tissues. On the other hand, the Ahr(-/-) mice appear to have a lower metabolic clearance of BP resulting in increased levels of DNA and protein adducts and of unmetabolized BP. In addition, we have performed an administration route experiment and found that skin-exposed Ahr(-/-) mice showed lower levels of protein adducts along with markedly reduced P450 1B1 expression, but only in the exposed area, as compared with the wild-type mice. In addition, the systemic uptake of BP is increased in the Ahr(-/-) mice as compared with the wild-type mice. Hence, the lack of a functional Ah receptor results in an Ahr-independent biotransformation of BP with a slower clearance of BP and higher levels of DNA and protein adducts, but the distribution and levels of BP and BP-protein adducts are clearly dependent on the route of exposure.

Coordinate regulation of Phase I and II xenobiotic metabolisms by the Ah receptor and Nrf2

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor with important roles in metabolic adaptation, normal physiology and dioxin toxicology. Metabolic adaptation is based on coordinate regulation of a set of xenobiotic-metabolizing enzymes (XMEs), termed AhR battery. Coordination is achieved by AhR/Arnt-binding to XREs (xenobiotic response elements), identified in the 5' upstream region of AhR target genes. The AhR battery encodes Phase I and II enzymes. Interestingly, these Phase II genes are linked to the Nrf2 gene battery that encodes enzymes that are essential in protection against oxidative/electrophile stress. Nrf2 binds to AREs (antioxidant response elements) in the regulatory region of a large and distinct set of target genes. Functionally characterized response elements such as XREs and AREs in the regulatory region of target genes may provide a genetic basis to understand AhR- and Nrf2-induced genes. Linkage between AhR and Nrf2 batteries is probably achieved by multiple mechanisms, including Nrf2 as a target gene of the AhR, indirect activation of Nrf2 via CYP1A1-generated reactive oxygen species, and direct cross-interaction of AhR/XRE and Nrf2/ARE signaling. Linkage appears to be species- and cell-dependent. However, mechanisms linking XRE- and ARE-controlled Phase II genes need further investigation. Tightened coupling between Phases I and II by AhR- and Nrf2-induced XMEs may greatly attenuate health risks posed by CYP1A1-generated toxic intermediates and reactive oxygen species. Better recognition of coordinate Phase I and II metabolisms may improve risk assessment of reactive toxic intermediates in the extrapolation to low level endo- and xenobiotic exposure.

Structure elucidation of phase II metabolites by tandem mass spectrometry: an overview

The present paper provides a summary of the collision-induced dissociation of protonated and deprotonated phase II metabolites of drugs and pesticides. This overview is based on published literature and unpublished data from the authors. In particular, glutathione conjugates and their biotransformation products are discussed in detail. In addition, the fragmentation of the major classes of conjugates, i.e. glucuronides, glucosides, malonylglucosides, sulfates, acetates, methyl and glycine conjugates, is reported. Collision-induced dissociation, as studied by tandem mass spectrometry, allows the rapid identification of the type of conjugate, whereas the exact conjugation site can in general be determined only by additional NMR experiments.

UDP-glucuronosyltransferase 1A6: structural, functional, and regulatory aspects

Glucuronidation, catalyzed by two families of UDP-glucuronosyltransferases (UGTs), represents a major phase II reaction of endo- and xenobiotic biotransformation. UGT1A6 is the founding member of the rat and human UGT1 family. It is expressed in liver and extrahepatic tissues, such as intestine, kidney, testis, and brain, and conjugates planar phenols and arylamines. Serotonin has been identified as a selective endogenous substrate of the human enzyme. UGT1A6 is also involved in conjugation of the drug paracetamol (acetaminophen) and of phenolic metabolites of benzo[a]pyrene (together with rat UGT1A7 and human UGT1A9). High interindividual variability of human liver protein levels is due to a number of influences, including genetic, tissue-specific, and environmental factors. Evidence shows that homo- and heterozygotic expression of UGT1A6 alleles markedly affects enzyme activity. HNF1 may be responsible for tissue-specific UGT1A6 expression. Multiple environmental factors controlling UGT1A6 expression have been identified, including the pregnane X receptor, the constitutive androstane receptor, the aryl hydrocarbon receptor, and Nrf2, a bZIP transcription factor mediating stress responses. However, marked differences have been noted in the expression of rat and human UGT1A6. Regulatory factors have been studied in detail in the human Caco-2 colon adenocarcinoma cell model.

Identification of 1-hydroxypyrene glucuronide in tissue of marine polychaete Nereis diversicolor by liquid chromatography/ion trap multiple mass spectrometry

1-Hydroxypyrene glucuronide is identified as the single major aqueous metabolite of the tetracyclic aromatic hydrocarbon pyrene, in tissue from a deposit-feeding marine polychaete, Nereis diversicolor. Identification was performed using an ion trap mass spectrometer fitted with an atmospheric pressure chemical ionization (APCI) probe and connected to a high-performance liquid chromatography/diode array detector (HPLC/DAD) system. Besides 1-hydroxypyrene, the 339-nm UV trace of tissue samples from pyrene-exposed worms showed only one dominant peak that could be related to pyrene metabolism. Negative APCI-MS of this supposed 1- hydroxypyrene conjugate gave a characteristic signal at m/z 429 corresponding to the molecular ion of 1-hydroxypyrene glucuronide plus eluent adducts ([M - H + 2H(2)O](-)). Fragmentation pathways were studied by isolating the abundant ion at m/z 429 in the ion trap and performing multiple mass spectrometric experiments (MS(n)). The fragmentations observed were consistent with the proposed identification. Two low intensity LC peaks that could be related to pyrene metabolism by their DAD absorption spectra were also present in the 339-nm UV chromatogram of tissue samples. However, these peaks could not be identified by their mass spectra in negative ion mode due to ion suppression by very abundant co-eluting impurities. The present method shows that LC/MS(n) is a fast and useful analytical tool for identification of aqueous polycyclic aromatic hydrocarbon biotransformation products in samples from relatively small marine invertebrates with limited sample preparation.

Ring opening of benzo[a]pyrene in the germ-free rat is a novel pathway for formation of potentially genotoxic metabolites

The metabolism of benzo[a]pyrene (BP) is known to lead to a large number of oxygenated compounds, some of which can bind covalently to DNA. We have studied the integrated metabolism of BP in vivo in germ-free rats given (14)C-labeled BP. Urinary metabolites were separated into groups according to acidity using lipophilic ion exchangers. The groups were analyzed by mass spectrometry and were further fractionated by high-performance liquid chromatography. The fraction of urinary metabolites previously shown to contain N-acetylcysteine and glucuronic acid conjugates was found to contain derivatives of 7-oxo-benz[d]anthracene-3,4-dicarboxylic acid as major components. These compounds, which were identified by mass spectrometry and NMR, accounted for about 30% of the total metabolites in urine, demonstrating that, surprisingly, ring opening is a major pathway for metabolism of BP in the germ-free rat. The dicarboxylic acid may be excreted in urine as an ester glucuronide. By using the single cell gel electrophoresis or COMET assay, we were able to demonstrate that the anhydride of 7-oxo-benz[d]anthracene-3, 4-dicarboxylic acid was an efficient inducer of DNA damage. Taken together, these results indicate that the novel ring opening metabolic pathway may provide alternative mechanisms for the toxicity of BP.